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Proton_Higher_Moments_3GeV

Updated on Mon, 2022-01-31 16:00. Originally created by sheppel on 2021-01-14 02:59.
Paper Title:
Measurements of Proton Higher Order Cumulants in \sqrtsNN  = 3GeV AuAu Collisions  and Implication of the QCD Critical Point

PAs:
Samuel Heppelmann, Yu Zhang, Daniel Cebra, Xin Dong, Xiaofeng Luo, Toshihiro Nonaka, Sooraj Radhakrishnan, Nu Xu

Targeted Journal:  
 PRL (short paper), PRC (long paper)

Abstract:
We report a new STAR measurement of cumulants of event-by-event proton multiplicities for the first dedicated fixed target physics run in most central (0--5\%) $\sqrtsNN=3.0$ GeV  \auau{} collisions. At central collisions, we observe the proton cumulant ratio of $C_4/C_2=-0.85 \pm 0.09 ~(\rm stat.) \pm 0.40 ~(\rm syst.)$ in $-0.5 < y<0$ and $0.4 < \ppt <2.0 $ GeV/$c$, significantly smaller than unity, the Poisson fluctuation baseline. The hadronic transport UrQMD model reproduces our data.Compared to higher energy results, the strong suppression in ratio of $C_4/C_2$ at 3\,GeV indicates an energy regime dominated by hadronic interactions. The QCD critical point, if discovered in heavy ion collisions, could only exist at energies higher than 3\,GeV.
 

Conclusions:
  • The first STAR measurement in a high baryon regime. At central collisions, we observe the $\sqrtsNN=3.0$ GeV \auau{} cumulant ratio of $C_4/C_2=-0.85 \pm 0.09 ~(\rm stat.) \pm 0.40 ~(\rm syst.)$ in rapidity window $-0.5<y< 0$.
  • Proton cumulant ratios agree with canonical ensemble (CE) model calculations for both rapidity regions $|y|<0.1$ and $-0.5<y<0$, namely a suppression of high order correlations due to baryon conversation. This differs from the expected ratio from a grand canonical ensemble (GCE) which is a flat function of collision energies. The large suppression suggests baryon conservation dominates the dynamics in the high baryon region. 
  • The UrQMD model calculations of proton multiplicity distribution are consistent with data from the highest ($\geq 50$ GeV) and lowest ($3$ GeV) collision energies. At energies from $\sqrtsNN=7.7$ to 27 GeV, there is a clear deviation from model calculations which do not include critical dynamics. 
  • Cumulant ratios at $\sqrtsNN=3.0$ GeV \auau{} collisions show no evidence of critical phenomena. The large negative value of the forth order cumulant ratios simply tells us that the baryon conservation dominant the dynamics in the high baryon region. On one hand, this finding confirms that the oscillation pattern~\cite{Adam:2020unf} at higher collision energy is beyond the dynamics in non-critical models such as UrQMD. On the other hand, it implies that the signatures of a critical point or phase boundary can only occur in higher energy region 3$< \sqrtsNN <$20 GeV.


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